With the ongoing miniaturization of electrical components and portable electronic devices in the last few years a new product category of so-called “wearables” has emerged, which comprises wearable wireless electronic devices with sensors for measuring biometric properties of a person wearing such a device. Fitness bands (e.g. fitness bands by fitbit Inc. of San Francisco, Calif., electronic watches (e.g. the Apple Watch by Apple Inc. of Cupertino, Calif.) and wearable wireless medical devices are examples of such wearables with biometric sensors.
In particular, some of these known devices are equipped with one or more sensors for measuring the composition and/or flow of blood in the body of the person wearing the device. The measurement of the composition of blood might for example relate to a measurement of levels of oxygen, nutrients or metabolic waste products in the blood. Measurements of the flow of blood might for example relate to the pulse at which the blood is pumped by the heart through the body. One way of performing such measurements is by using optical means provided within the wearable device, wherein a light source (such as an LED) is used to emit a beam of radiation, in particular light in the visible or infrared part of the electromagnetic spectrum, towards and under the skin of the person. An optical sensor (such as a photodiode) is then used to detect the radiation reflected from the body, in particular from blood flowing in the blood vessels below the skin. The spectral response of the detected reflected radiation can be analyzed in order to derive the biochemical composition of the blood, because the various types of molecules to be detected within the composition of the blood have specific individual spectral responses to the radiation emitted by the light source. Therefore, the optical sensor has to be able to distinguish different wavelengths of the spectrum. To achieve this, filters can be used, in particular narrow filters with a bandwidth of about 50 nm or less. In order to detect the flow of blood, esp. its pulse, variations over time of the pulse-dependent intensity of the detected responses can be measured. In particular, the intensity is dependent on the oxygen level in the blood and thus serves as a good indicator for the pulse, as the oxygen content of blood at a particular vessel location usually varies, at least roughly, with the rhythm of the pulse.
Many wearable devices with biometric sensors, in particular if they are designed as watches or fitness bands, are worn at an extremity of the body, mostly at the wrist, and may have a bracelet to fix them thereto. While pulling the bracelet rather tight enables a higher quality of measurement, because then one or both of the lateral position and the distance of the sensor relative to the skin are less likely to change when the body moves, this is often considered very inconvenient for the person wearing the device. Thus, despite their intended biometric functionality, such wearable devices are often worn rather loosely thus reducing measurement quality or even preventing of meaningful measurements. This in turn might lead to a reduction of acceptance of such devices by users and ultimately to a reduced use rate or even a termination of use. Similar problems can also occur, when the wearable device is not to be worn at the wrist but is designed to be in some other way coupled to the body, e.g. at other parts of the extremities or around the chest (e.g. by a chest strap).
In the field of wired medical devices, in particular oximeters, it is known to use several sensors and to switch between them to manually or automatically and even dynamically select only one of the various measured sensor outputs as input signal to the medical device.
Such a solution is known from U.S. Pat. No. 6,510,331 to Williams et al., which discloses a switching device being interposed between a conventional oximeter and a plurality of conventional photo sensors. Photo sensors are located on different extremities of the body. The switching device may be operated in a manual mode or an automatic mode. In the manual mode, each different photo sensor may be individually selected to provide the input signal to the oximeter. In the automatic mode, the switching device scans the incoming signals from the different photo sensors and forwards the best, strongest or least distorted signal to the oximeter. The device prevents the loss of oximetry information due to interrupted blood flow in a particular part of the body or the failure of a sensor.